Issue #2493091 by Wim Leers: Installing block module should invalidate the 'rendered' cache tag

core/modules/block/block.module

@@ -7,6 +7,7 @@
 
 use Drupal\block\BlockInterface;
 use Drupal\Component\Utility\Html;
+use Drupal\Core\Cache\Cache;
 use Drupal\Core\Routing\RouteMatchInterface;
 use Drupal\Core\Url;
 use Drupal\language\ConfigurableLanguageInterface;
@@ -306,3 +307,13 @@ function block_configurable_language_delete(ConfigurableLanguageInterface $langu
     }
   }
 }
+
+/**
+ * Implements hook_install().
+ */
+function block_install() {
+  // Because the Block module upon installation unconditionally overrides all
+  // HTML output by selecting a different page display variant, we must
+  // invalidate all cached HTML output.
+  Cache::invalidateTags(['rendered']);
+}

core/modules/block/src/Tests/BlockInstallTest.php

+<?php
+
+/**
+ * @file
+ * Contains \Drupal\block\Tests\BlockInstallTest.
+ */
+
+namespace Drupal\block\Tests;
+
+use Drupal\simpletest\WebTestBase;
+
+/**
+ * Tests block module's installation.
+ *
+ * @group block
+ */
+class BlockInstallTest extends WebTestBase {
+
+  public function testCacheTagInvalidationUponInstallation() {
+    // Warm the page cache.
+    $this->drupalGet('');
+    $this->assertNoText('Powered by Drupal');
+    $this->assertNoCacheTag('config:block_list');
+
+    // Install the block module, and place the "Powered by Drupal" block.
+    $this->container->get('module_installer')->install(['block', 'shortcut']);
+    $this->rebuildContainer();
+    $this->container->get('router.builder')->rebuild();
+    $this->drupalPlaceBlock('system_powered_by_block');
+
+    // Check the same page, block.module's hook_install() should have
+    // invalidated the 'rendered' cache tag to make blocks show up.
+    $this->drupalGet('');
+    $this->assertCacheTag('config:block_list');
+    $this->assertText('Powered by Drupal');
+  }
+
+}